Addiction is a condition that results when a person ingests a substance (e.g., alcohol, cocaine, nicotine) or engages in an activity (e.g., gambling, sex, shopping) that can be pleasurable but the continuation of which becomes compulsive and interferes with ordinary responsibilities and concerns, such as work, relationships, or health. People who have developed an addiction may not be aware that their behavior is out of control and causing problems for themselves and others.

The word addiction is used in several different ways. One definition describes physical addiction. This is a biological state in which the body adapts to the presence of a drug so that drug no longer has the same effect, otherwise known as a tolerance. Another form of physical addiction is the phenomenon of overreaction by the brain to drugs (or to cues associated with the drugs). An alcoholic walking into a bar, for instance, will feel an extra pull to have a drink because of these cues.